1. Field of the Invention
The present invention relates to a digital non-linear pre-emphasis/de-emphasis apparatus and, more particularly, to a digital non-linear pre-emphasis/de-emphasis apparatus suitable for a system which requires a filter that non-linearly changes the system frequency response characteristic according to the input signal amplitude.
Korean Patent Application No. 93-4987 is incorporated herein by reference for all purposes.
2. Brief Discussion of Related Art
Generally, a non-linear pre-emphasis/de-emphasis apparatus is used for a video cassette recorder (VCR) to enhance the signal-to-noise (S/N) ratio of a high frequency signal during recording and reproduction.
Pre-emphasis is a process by which the higher frequency components of a baseband signal are emphasized prior to recording or transmission to offset loss in the S/N ratio of the high frequency component of the baseband signal decreased during recording/reproduction or transmission/reception. It will be noted that de-emphasis is a process by which the high frequency components emphasized for recording or transmission are restored to their original waveform during reproduction or reception.
In the case of a VCR, the high frequency component of the input signal is emphasized by a non-linear pre-emphasis apparatus during recording while, the emphasized high frequency component is attenuated by means of a non-linear de-emphasis apparatus during reproduction. The operational characteristics of the non-linear pre-emphasis apparatus mirror those of the de-emphasis apparatus, so that signal gain does not change throughout recording and reproduction.
In a VCR wherein analog signals are being processed, the non-linear characteristic of a diode is utilized in constructing the non-linear pre-emphasis/de-emphasis apparatus. This analog non-linear pre-emphasis/de-emphasis apparatus is generally classified into a non-linear pre-emphasis/de-emphasis portion and a main pre-emphasis/de-emphasis portion.
In the first generation of video home system (VHS) format VCRs, a non-linear pre-emphasis/de-emphasis was not used; only the main pre-emphasis/de-emphasis portion was used. Second and third generation VHS format VCRs, in general, include two portions: a non-linear pre-emphasis/de-emphasis portion and a main pre-emphasis/de-emphasis portion. A conventional analog non-linear pre-emphasis apparatus is shown in FIG. 1, in which the apparatus includes a non-linear pre-emphasis portion 12 having a resistor R1 and a capacitor C1 which are connected in parallel with respect to each other to obtain a first impedance value Z1, two diodes D1 and D2 connected in parallel with respect to each other and between a capacitor C2 and ground, capacitor C2 being connected in parallel with a resistor R2, one end of which is grounded, for obtaining a second impedance value Z2. An adder 14 is connected to non-linear pre-emphasis portion 12 for receiving an input signal S.sub.IN for adding image signal S.sub.IN to the output signal of non-linear pre-emphasis portion 12. A main emphasis portion 16 generates the image signal with improved S/N ratio with respect to the output from adder 14. It will be noted that during operation of non-linear emphasis portion 12 shown in FIG. 1, the non-linear characteristic of diodes D1 and D2 permits conduction of high-amplitude input signal S.sub.IN, so that capacitor C2 is effectively grounded.
Since first impedance Z1, which is formed by capacitor C1 and resistor R1, and second impedance Z2, which is formed by capacitor C2 and resistor R2, are equal to one another, the output of non-linear pre-emphasis portion 12 becomes flat and constant for the entire operational bandwidth.
On the other hand, when a low-amplitude input signal S.sub.IN is applied, diodes D1 and D2 do not conduct and, since they are essentially acting as an open circuit, effectively remove capacitor C2 from non-linear pre-emphasis portion 12.
Accordingly, first impedance Z1, which is formed by capacitor C1 and resistor R1 in non-linear pre-emphasis portion 12, varies with frequency. Adder 14 adds input signal S.sub.IN to the output signal of non-linear pre-emphasis portion 12, so that the amplitude of the output signal of adder 14 becomes larger for higher frequency signals so that only the high frequency input signal having a low amplitude is boosted by about 5-6 dB.
The frequency versus gain characteristic of non-linear pre-emphasis portion 12 varies according to the amplitude of the input signal, as shown in FIG. 2A. That is, at higher input signal amplitudes the degree of high frequency emphasis is reduced, while at lower input signal amplitudes the degree of high frequency emphasis is increased.
Non-linear pre-emphasis portion 12 improves the S/N ratio of the high frequency components of the video signal, which greatly affects image resolution and, thus, improves the S/N ratio and the contours, i.e., distinctness, of the reproduced image.
On the other hand, main pre-emphasis portion 16 changes the frequency characteristic of the signal output from adder 14 by varying an internal time constant. Main pre-emphasis portion 16 prevents degradation of the S/N ratio in a reproduced video signal during demodulation, e.g., the signal frequency modulated during recording or transmission. The frequency versus gain characteristic of main pre-emphasis portion 16 is shown in FIG. 2B. The frequency characteristic of the emphasized signal S.sub.EMP output by main emphasis portion 16 is determined according to the time constant.
Therefore, in VCRs where the luminance signal is frequency modulated prior to being recorded, pre-emphasis processing emphasizes the high frequency band whereby its amplitude is increased. When de-emphasis is performed after frequency-demodulation, by circuitry having the opposite characteristic to that of the pre-emphasis performed before modulation, the amplified high frequency band is restored to the original signal shape.
In recent years, part or all of the signal processing conventionally performed by analog circuitry is more frequently being performed by digital domain processing, for stabilizing and, thus, improving the S/N ratio of the VCR system.
When operating in the recording mode, the input analog video signal is converted into a digital signal and a variety of digital signal processing operations are performed during the recording process. It will be noted that digitized signal is converted back into an analog signal before being ultimately recorded onto the magnetic tape.
In contrast, during the reproducing mode, the analog video signal, after being reproduced from the magnetic tape, is converted into a digital signal which is then subjected to a variety of digital signal processing operations. Again, the digitized signal is converted back into an analog signal before being ultimately used, e.g., output as video signal to a display device. When the video signal is processed in digital form during the record/reproducing mode, the non-linear pre-emphasis and de-emphasis operations having opposite characteristics with respect to each other are performed by digital processing.
One digital non-linear emphasis/de-emphasis apparatus is disclosed in U.S. Pat. No. 4,849,826, wherein the passband of a high-pass filter for extracting the higher frequency components from the input signal varies according to the input signal characteristic and wherein feedback is provided from the output signal thereof. In this apparatus, non-linear input and output circuits having different coefficients of non-linearity are separately constructed for emphasis and de-emphasis. Input signal frequency characteristics must first be controlled before the gain is controlled. Therefore, the low frequency components having a small gain of the input signal are less emphasized than the high frequency components having a large gain thereof so that the gain of the low frequency components are not properly controlled, which inhibits accurate emphasis/de-emphasis control of the low frequency components with a small gain of the input signal.
Furthermore, a digital non-linear pre-emphasis technique is disclosed in U.S. Pat. No. 4,668,988, wherein the non-linear pre-emphasis circuit is constructed digitally such that system volume can be reduced while performance is enhanced. In the circuit, a desired frequency characteristic of a digital video signal is obtained by using a digital filter comprised of an adder, a delay, a counter and a subtractor. In addition, a ROM holding a data conversion table for non-linearly suppressing the amplitude of the digital video signal is utilized, so that data, whose amplitude corresponds non-linearly to that of the input signal, is added to the input digital video signal, which thereby results in an emphasis processing with respect to the amplitude of the input signal.
During pre-emphasis, input signals of the same frequency and amplitude should have equal frequency characteristics, without reference to a DC offset level. However, the above-described circuit exhibits different frequency response characteristics according to variation in the DC offset level.